The ~(176)Lu decay constant determined by Lu–Hf and U–Pb isotope systematics of Precambrian mafic intrusions

摘要

At present, there is large uncertainty in the decay constant of ~(176)Lu needed for Lu–Hf isotopic studies. We have determined λ~(176)Lu by cross-calibration of U–Pb and Lu–Hf isotopic systems on mineral fractions from the Proterozoic Karlshamn and Sorkka dolerites in Sweden and Finland. The dolerites crystallized at shallow depths from homogeneous, high-temperature magmas, carry olivine that is nearly 100% unaltered, and show no signs of post-magmatic isotopic disturbance. The Lu and Hf isotopic compositions of plagioclase, olivine, pyroxene, apatite, ilmenite and baddeleyite were determined by multicollector-inductively coupled plasma mass spectrometry (MC-ICPMS). Calibrating the Lu–Hf results against baddeleyite U–Pb dates of 954.1±1.2 and 1256.2±1.4 Ma for the dolerites yields a mean 176Lu of 1.867±0.008×10-11 year-1. The pristine character of the rocks and the agreement of the λ~(176)Lu values with those from other terrestrial data sets [E. Scherer et al., Science 293 (2001) 683–687] suggest that the true value of λ~(176)Lu lies between 1.86 and 1.87×10~(-11) year~(-1). Calibration experiments on extraterrestrial samples give significantly higher (4–6%) values, a discrepancy that may be due to plotting of non-cogenetic samples on the same Lu–Hf isochron diagram, or may have other, as yet undetermined, causes. The result of this study also indicates that the Lu–Hf method is capable for dating the crystallization of mafic rocks. The high 176Lu/177Hf ratio in apatite suggests that intrusive ages can be determined at a precision of a few million years or better.